1. Field of the Invention
The present invention relates to recovery of delta-phase plutonium metal.
2. Discussion of Background
Plutonium metal is produced in six different crystalline phases. Some of these phases, such as the so-called alpha phase are stable; others, such as the delta phase are not stable without alloying with another metal such as aluminum or gallium. Delta phase plutonium is less dense than the more common alpha phase and it is two or three times more reactive, but delta phase plutonium is more machinable. Delta plutonium, because it is more machinable, finds use in a number of important defense-related applications. However, after a period of time during which radioactive decay introduces gamma-emitting impurities, the delta phase plutonium must be recovered and reprocessed before reuse.
Typically delta-phase plutonium purification begins by dissolving the plutonium metal in sulfamic acid. Dissolution is followed by separation of impurities, concentration, precipitation, filtering and drying to produce alpha-phase plutonium metal. However, the dissolution step generates a stoichiometric yield of hydrogen that must be handled carefully lest its concentration approach the explosive limit. This first step can also produce substantial quantities of plutonium hydride which, being pyrophoric, can spontaneously ignite during servicing of the dissolution vessel. Finally, a substantial amount of sulfate is generated by the oxidation or hydrolysis of the sulfamic acid. Sulfate is believed to be a principal cause of corrosion of waste evaporators.
Other processes related generally to plutonium recovery are known. Horwitz, et al., in U.S. Pat. No. 4,162,230, describe a process for recovery of actinide elements from nuclear reactor wastes, including plutonium. Their process includes adding hydrazine and hydroxylammonium nitrate (about 0.3 M each) to the waste to adjust the valence of the plutonium to 30 4. The hydrazine reduces any excess HNO.sub.2 present in the solution which would destroy the HAN.
Hausberger, et al. describe another method for purifying plutonium in U.S. Pat. No. 4,021,313. Plutonium III in a solution of hydrazine nitrate and HAN is subjected to an electrolysis charge to oxidize anodically he plutonium III to plutonium IV.
Germain, et al., in U.S. Pat. No. 4,983,300, describe a process for the reextraction of plutonium present in an organic solvent by reducing plutonium IV to plutonium III. The reduction is achieved by contacting the solvent with an acidic mixture of a uranous salt and hydroxylamine salt.
Other references describe processes for concentration and separation of actinides (Horwitz, et al., in U.S. Pat. No. 4,835,107), for extraction of plutonium (Bathellier, et al., in U.S. Pat. No. 4,7674,352; Horwitz et al., in U.S. Pat. No. 4,574,072), for stripping plutonium from organic solvents (Bathellier, et al. U.S. Pat. No. 3,981,961), and for partitioning uranium and plutonium. Chapman, et al. (U.S. Pat. No. 4,229,421) describe a method for purifying plutonium contained in an aqueous solution along with uranium and neptunium.